UNCLASSIFIED AD D FENSIE DOCUMENTATION CENTER FOR SCIIENI'FIC AND TECHNICAL INFORMATION VIRGINIA CAMEION STATION, ALEXANDRIA, UNCLASSIFIED

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1 UNCLASSIFIED AD D FENSIE DOCUMENTATION CENTER FOR SCIIENI'FIC AND TECHNICAL INFORMATION CAMEION STATION, ALEXANDRIA, VIRGINIA I UNCLASSIFIED

2 / Space Sciences Laboratory University of California Berkeley 4, California / MM RADIO EMISSION FROM JUPITER AND VENUS 13 by 0;?e " D. D. Thornton and W. J. Welch LA NSF Grant NSF G i' " and ONR Contract Nonr Series No. 4, technical report on research supported by ~ Q ssue No. Z 5 Reproduction in whole or part is permitted for any purpose of the U. S. Government. April 16, 1963

3 mm Radio Emission from Jupiter and Venus by D. D. Thornton Space Sciences Laboratory University of California, Berkeley and W. J. Welch Department of Electrical Engineering and Space Sciences Laboratory University of California, Berkeley Support for this work has been provided by the Office of Naval Research through contract Nonr 222(54) and the. Atmospheric Sciences Program of the National Science Foundation, grant NSF G

4 / 3 Summary Observations of the mm radio emission from Jupiter and Venus are reported. A disc temperature of Y is found for Jupiter and a temperature of O is found for Venus. is shown that the; 8, mm brightness temperature of Jupiter should correspond clopely to the temperature at the level of the visible Scloud cover. It i;

5 mm Radio Emission from Jupiter and Venus Introduction A number of observations of the millimeter radio emission from Jupiter were made following the planet's opposition at the end of August, A study of the emission of Venus at, this wavelength was begun simultaneously, to be carried out through conjunction in November and beyond. The intent of the latter program is the investigation of possible variations of the planet's brightness with either phase angle or time. The results of the J,..upiter observations are presented here as well as preliminary results for Venus. - Procedure The antenna used for these-measurements is a ten:foot diameter aluminum rf flector cast and machined at the U. S. Naval Shipyard in Norfolk, Virginia. The input section of the receiver is located behind the feed horn at the focus in order to avoid feed losses. The gain of the antenna has been measured by locating a transmitter on a nearby hill and comparing the signal received by the antenna with that of a standard gain horn. Because of the height of the hill and that of the antenna above ground and the narrow radiation pattern of the transmitter, no errors due to reflections from the ground are expected. probable error of 10 per cent. The gain is 5 x 105 with a The radiometer is of the type first described by R. H. Dicke, The rapid comparison is provided by a ferrite microwave switch at the input. The reference termination consists of a small horn antenna oriented in the same direction as the large antenna, so that the reference temperature, provided by atmospheric absorption, is very close to that of the large antenna. In this circumstance, the effect of changes in receiver gain. on the accuracy of the measurements is small. The beamwidth of the reference horn is large enough (200) so that even the presence of the sun within the beam barely alters its temperature. The receiver noise temperature is 25000, including 0. 4 db insertion loss in the ferrite switch, and its bandwidth is 30 megacycles. The output fluctuation with a 5 second integration time amount to about L degree peak to peak or about RMS. A calibration signal is provided by means of a small

6 5 amount of noise fed into the receiver through a directional coupler from an argon discharge tube. This signal has been calibrated by comparison with the signals emitted by two matched terminations at different temperatures which were connected to the input of the radiometer in succession. The temperatures of the terminations were regulated and measured to within C. They differed by about 20 0 C. The gain stability of the radiometer is 0o 1 % for 30 minute periods and about 1 % for longer intervals. The manner of observation consists of pointing the antenna West of the planet anad allowing the planet to drift through the beam with the antenna stationary, Accurate positioning is accomplished by means of an optical sighting telescope mounted on the edge of the antenna. sighting telescope was aligned during the gain calibration. Attempts to make measurements by alternately tracking the planet and an empty sector of sky nearby proved unsatisfactory due to the effects of atmospheric absorption at this wavelength, Escept when the planet is near the meridian, its zenith angle changes rapidly enough during any suitable tracking period to make the background sky temperature change significantly. The Although this effect is somewhat reduced through the use of the reference horn pointing in the same direction as the main antenna, it remains troublesome. Results for Jupiter Because the emission from Jupiter is relatively weak, it could not be seen on individual drift scans, Figure 1 shows the result of averaging 16 scans taken on September 18 and 20. The resulting antenna temperature due to Jupiter is 0, 20 0 K. The atmospheric absorption was measured and found to be 5% at zero zenith angle, This absorption was inferred from a measurement of antenna temperature as a function of zenith angle. With atmospheric absorption and the slight smoothing of the trace due to the 5 second time constant accounted for, the disc brightness temperature implied by the September 18 and 20 observations is Further measurements were made on October 1, 4, 29, and 30. The average disc temperature for 16 scans taken on October 1 and 4 is 1340 and for 32 scans taken on October 29 and 30 is 1460, The average for all 64 scans is The probable error due to the residual noise on the traces is 180. When the systematic error, primarily due to uncertainty in antenna gain, is

7 6 added, the total probable error is 230. Measurements of Jovian radiation between 3 and 68 cm exhibit a spectrum which differs markedly from that of a black body, the flux density being nearly constant between 10 and 68 cm. These measurements as we]k as various explanations of them that have been advanced are summarized by Drake, 1961; Mayer, 1961; and Burke, The spectrum of this longer wavelength radiation as well as its polarization properties and the fact that it appears to fluctuate suggest that it arises from an electron gas surrounding the planet. The disc temperature at 3 cm has been found to be approximately As pointed out by Drake, 1961, a large part of this probably arises as black body emission from the planetary surface or lower atmosphere. be entirely of this nature. We should therefore expect the radiation at 8 mm to The. available information concerning the atmosphere of Jupiter is summarized by Kuiper, 1952, and more recently by Opik, In his discussion Kuiper proposes an atmosphere rich in hydrogen and helium with an isothermal stratosphere at 860 extending down to a troposphere in adiabatic equilibrium with the corresponding constant temperature gradient. The visible cloud layer occurs at some level within the troposphere. amounts of ammonia and methane are observed above the clouds. amount of ammonia above the clouds which is inferred from the observed spectrum of sunlight reflected from the clouds is 700 cm-atmospheres at STPo At low temperatures, the vapor pressure of ammonia is quite low, and Kuiper furtler supposes that the partial pressure of the ammonia at any level above the clouds is limited by the vapor pressure at the correspc- ling temperature at that level. with the solid phase which forms the visible clouds. The The ammonia vapor is in equilibrium Small The requirement that the visible atmosphere contains the observed. abundance of ammonia then fixes the cloud top temperature at about Direct evidence that the visible atmopshere is primarily composed of hydrogen and helium is provided by the observation by Baum and Code, 1953, of the occultation of a- Arietis by Jupiter. They concluded that the mean molecular weight lay in the range of 2. 2 to 4. 4 for a stratospheric temperature of 860..

8 7 For the purpose of comparison, we shall estimate the expected planetary disc temperature at mm on the basis of Kuiper's model b atmosphere. This atmosphere contains per cent helium and per cent hydrogen, the remainder including ammonia and methane. The mean molecular weight is 3. 26, and the mean ratio of specific heats is The corresponding adiabatic temperature gradient is 4.00 per kilometer. The cloud top temperature is 1680, and the pressure there is 2 atmospheres. Ammonia is the one constituent. which has appreciable absorption at microwave frequencies due to its inversion band centered at 1., cm, so that the 8.35 mm emission will arise from this component alone. As its partial pressure above the clouds is limited by its vapor pressure, its abundance will decrease very rapidly above the clouds with the uniform decrease in temperature. We may therefore ignore the stratosphere and suppose that the adiabatic atmospher extends out to its natural limit of 42 kilometers. The temperature at a height x above the clouds is T -- (i> where T c 1680 and h = 42 km. From the discussion above and the Clausius-Clapeyron equation, the ammonia de sity above the clouds is ncce (2) Te where nc is the density at the cloud tops, chosen so that the total abundance is 700 cm-atmospheres. This makes the relative abundance at cloud tops 1. 2 x The half widths of the individual ammonia lines are given by (Birnbaum and Maryott, 1954) J K 3

9 8 Among the stronger lines the range of half widths is small, and we shall adopt the uniform value of 25 mcs. at 1 mm pressure and T = K. From the assumed abundances of hydrogen and helium :and the pressure broadening data of table 13-4 of Townes and Shawlow, 1955, the line width at the cloud tops is 5. 1 kmc. We assume that the temperature dependence of AV in the mixture is given by (3). Above the clouds the half width is therefore A1 D5 (4) T The absorption at any frequency V due to any line m is given approximately by (Townes and Schawlow, 1955, p. 343) where r is the frequency of the line and the peak intensity ' is "given in table 12-3 of Townes and Schawlow for pure ammonia. In the mixture( AV ) remains fixed, so that the peak line intensity at the cloud tops is I, 5 #0"O Above the clouds the peak intensity varies as T ". The total absorption is the sum of contributions from about 24 strong lines. The optical depth down to a height X abovye the clouds is given by T (6) Combining these relations, we have 74 _ (8)

10 9 provided that (V-)) bv2). For i =36 kmc. ( \ = 8.35mm), the value of the sum is found from table 12-3 of Townes and Schawlow to be x I03. Therefore, 44,(- ) - -,) (,Y) 3,7 =3.7e, (9) As the total optical depth down to the clouds is 3. 7, we expect the emission temperature to correspond to a level slightly above. The planetary disc temperature may be estimated as follows. Rewrite (9) so that 7_'"D (10) The brightness temperature of a pencil of radiation with the direction cosineu relative to the normal due to the ammonia above the clouds is except near 0. The disc tempera t ure is then ~ ~W 0O(12) The predicted value of 1600 is just within the probable error of the observed temperature of If one were to assume a greater relative abundance of hydrogen to helium, as in the case of Kuiper's model a, 1952, a slightly greater absorption at mm would be predicted due to increased pressure broadening. However, the emission temperature would be only slightly changed because the ammonia exists in a very thin layer as indicated by equation (2). In fact, the mm temperature should be close to the cloud top temperature for a range of possible atmospheres, so long as: (a) the ammonia is in saturation equilibrium with the sqlid phase, (b) the amount above the clouds is "about 700 cm-atmospheres, and (c) the total pressure at the cloud tops is at least 2 atmospheres, with the atmosphere mostly consisting of I/ hydrogen and helium. Ir his recent discussion of Jupiter, Opik, 1962,

11 10 has concluded that the cloud top temperature is 1560 and the pressure about 11 atmospheres, the main constituent of the visible atmosphere being helium. The mm temperature for this model is about 1500 which is fairly close to the observed val'e of 1440 reported here. Finally, it should be noted that the requirement of saturation equilibrium for the ammonia implies that there be solid ammonia particles throughout the extent of the gas phase, so that'the "cloud tops" do not foi.m a single reflecting layer for visible light. The inferred ammonia abundance of 700 cm-atmospheres (Kuiper, 1952, p. 374) based on a simple reflecting layer model may therefore be somewhat. in error. Results for Venus Venus was first observed on October 17. An average of 16 scans on that day showed a mean increase in antenna temperature due to Venus of (Figure 2). The disc temperature implied by this is TO N3 ±00 Observations nearer conjunction yielded temperatures which are slightly higher but within the probable error. This result is not significantly different from the value of reported by Gibson and McEwan, 195.9, at 8. 6 mm and the value of 315 ± 700 reported by Kuzmin and. Salomonovich, 1960, at 8. 0 mm. Acknowledgement It is a pleasure to acknowledge the assistance of W. Caton and the advice and encouragement of Prof. S. Silver and Dr. D. Rea.

12 A 11 References BAUM, W.A., and CODE, A, D. (1953). A photometric observation of the occultation of o-arietis by Jupiter. Astron. J. 58, pp BIRNBAUM, G., and MARYOTT, A. (1953). Absorbtion in the lowfrequency wing of the NH 3 Inversion spectrum. J. Chem. Phys., 21, pp BURKE, B. F. (1961). Radio Observations of Jupiter. I. In "Planets and Satellites: The Solar System III,,, (ed. by G. P. Kuiper and B. M. Middlehurst), pp Univ. of Chicago Press, Chicago. DICKE, R. H. (1946). The Measurement of thermal radiation at microwave frequencies. Rev. Sci. Inst. 17, No. 7, pp DRAKE, F. (1961). Radio emission from the planets. Physics Today. 14, No. 4, pp GIBSON, J. E., and McEWAN, R. J, (1959). Proc. I. A. U. Symp., No. 9 - U. R. S. I. Symp. No. 1., (ed. R.N. Bracewell), Stanfoid University Press, pp KUIPER, G. P. (1952). Planetary atmospheres and their origin. In --The-Artrospheres of the Earth and Planets". (G. P. Kuiper, ed.), pp KUZMIN, A, D., and SALOMONOVITCH, A. E. (1960). A. J. (U. S. S. R.). 37, p MAYER, C, H. (1961). Radio emission of the moon and planets. In "Planets and Satellites: The Solar System III". (ed. by G. P. Kuiper and B. M. Middlehurst), pp University of Chicago Press, Chicago. 6PIK, E.J. (1962). Jupiter: chemical composition, structure, and origin of a giant planet, Icarus. 1, No. 3, pp Microwave Spectro- TOWNES, C. H,, and SCHAWLOW, A. L. (1955). scopy. McGraw-Hill, New York.

13 7 12 Figure 1. The average of 16 drift scans of Jupiter taken on September 18 and September 20. The ordinate is antenna temperature. Figure 2. The average of 16 drift scans of Venus taken on October 17, 1962, about four weeks before inferior conjunction. The ordinate is antenna temperature. it _

14 LaJOD w z 0 z Ix

15 tn- (0 LA-J zz 0 z

16 / Appendix I. Measurement of Atmospheric Absorption In order to relate the antenna temperature resulting from the presence of the planet in the antenna beam to the planetary disc temperature, the absorption of the earth's atmosphere must be known. Because the signals observed are so weak relative to the background receiver noise, it is difficult to measure the variation of the apparent brightness of the planet with zenith angle and then extrapolate to zero atmo-spheric air mass. If the sun is in the sky at the time of observation, it may be used for this purpose as it is a very bright source. In its absence the absorption is inferred from observations of the antenna temperature as a function of zenith angle with no extraterrestrial source in the antenna beam. T sec Z - T sec Z de Th T A(Z)dL = T(Tr)e d T sec ecz (A. 1) Z is the zenith angle, T(T) is the atmospheric temperature at any optical depth T above the ground, T 0 is the total (normal) optical depth of the atmosphere. Most of the absorption and consequent emission occurs in the troposphere where the temperature does not differ greatly from the ground temperature. The above equation may then be approximately written =-T sec Z T A(.Z) Lt <Ts) (.I - e -) e (A. 2) It is found that this equation fits observations closely with (T K. Thus a few observations of TA(Z) on any given day provide a value for T 0 In the present case these measurements are made as follows. reference horn is oriented at right angles to the main antenna in the meridian plane; that is, the main antenna but 900 greater declination. The the reference horn has the same hour angle as Then when the antenna is pointed 450 from the zenith in the meridian plane, both it and the reference horn have the same temperature.. A few readings of the temperature difference between the two for a few angles other than 450 then provide T. This procedure has the advantage of simplifying baseline

17 calibration. It also minimizes the effects of baseline drift due to receiver gain change. II. Signal Reduction Due to the Output Integrator Smoothing of the output noise of the radiometer is provided by a double-pole integrator, i. e., two identical simple R-C filters in cascade. When the planet passes through the antenna beam, the radiometer response is somewhat distorted due to the inertia of the integrating circuit. Naturally, this limits the amount of integration time that may be employed relative to the transit time of the source through the antenna beam. The reduction in the maximum response of the radiometer output for this twopole integrator as a function of the ratio of source transit time to RG has been calculated, * RG is the time constant of each pole; the five seconds referred to in the above text corresponds to each pole. The antenna beamwidth is minutes. With the five second time constant this gives a reduction in the maximum radiometer response of 5 per cent for Jupiter drifting through the beam on October 1, This value was used to correct the drift curves. Radiometer Studies and Related Problems, Space Sciences Laboratory Series 4, Issue 22,